The invention is directed to a process for the separation and purification of lactic acid from salt-containing and carbohydrate-containing substrates (fermentation solution) from which coarsely dispersed and lipophilic impurities have been removed.
Industrial production of lactic acids, especially where pure L(+)-lactic acid and D(-)-lactic acid are to be extracted, is presently carried out predominantly by biotechnological processes. The production process can be divided into:
1) the actual production of lactic acid by fermentation of a carbohydrate-containing medium;
2) the working up (downstream processing) of the fermentation solution to form pure acid. PA1 The fermentation mash need not be mixed with a strong mineral acid to liberate the lactic acid, which considerably reduces the salt contents of the mash. Compared with methods in which strongly acidic cation exchangers are used to separate the lactic acid from the usually acidified liquor by classic ion exchange, the chief advantage consists in that the amount of mineral acid required for regenerating a weakly acidic ion exchanger is reduced by roughly two thirds. PA1 The advantage compared with IEC methods with basic anion exchangers consists in that the lactic acid fraction is obtained as a sharp symmetrical peak without "tailing". PA1 Further, the separation of lactic acid from other present organic acids, in particular ethanoic acid, is also possible on strongly acidic ion exchangers. Accordingly, pharmaceutical-grade lactic acid with sufficient color stability can be obtained so that it can also be used for polymerization. PA1 In contrast to IEC methods with basic exchangers requiring diluted acids for elution, pure water can be used as eluting agent in the present process.
Industry has adopted a large number of strains of microorganisms for the production of lactic acid. The most important of these are the homofermentative lactic acid bacteria of the genera lactobacillus, streptococcus and pediococcus. However, these genera reach their maximum productivity only within a very narrow pH range.
Therefore, it is necessary during fermentation not only to maintain a constant optimum temperature for the selected organism, but also to maintain the required pH at a constant value. For this reason, neutralizing agents such as alkali hydroxide, calcium carbonate, milk of lime or ammonia water are added to the mash before and/or during fermentation so as to prevent over-acidification and to maintain a constant pH of 5.5 to 6.5.
Thus, the main component contained in the fermentation mash is the salt of lactic acid (e.g., NH.sub.4 lactate or Ca lactate) in addition to a little free acid, as well as unconverted starter materials (e.g., sugar), heavy metals, coloring matter, metabolic by-products (e.g., acetic or ethanoic acid), cells and cell fragments of the microorganisms, and inorganic salts.
Therefore, direct use of the solution coming from fermentation is not possible and further processing steps are required to extract the pure, free lactic acid.
A number of methods are described for the separation and extraction of free lactic acid from the fermentation mash.
The simplest commercial method of purification, and that most often applied by manufacturers, is precipitation of the lactic acid as calcium lactate. In this method, at the end of the fermentation process the mash is first heated to approximately 80.degree.-90.degree. C. and the pH is increased to 10-11. In this step, the microorganisms are destroyed, the proteins are coagulated and the formed calcium lactate is dissolved. After all insoluble components have been separated, the mash is acidified with sulfuric acid to liberate the lactic acid from its salt. In order to remove the iron and copper ions introduced especially as a result of corrosion, as well, sodium hexacyanoferrate (II) or calcium hexacyanoferrate (II) is added and the precipitated ferrocyanide salts, together with the calcium sulfate, are separated by means of a rotary filter or filter press. Coloring components are removed by activated charcoal. The obtained dilute acid is then concentrated to lactic acid of approximately 80% strength, small amounts of resulting volatile acids being removed at the same time.
In improved precipitation processes, the crude lactic acid is first decolorized with activated charcoal and subsequent purification steps can be carried out by means of cation exchangers for complete removal of any remaining salts. The cation-free solution can then be evaporated or crystallized immediately or can be guided through an anion exchanger in order to remove any remaining foreign anions, mainly sulfate ions and chloride ions (DD-PS 6740).
For a further improvement in quality, especially with respect to odor and flavor, an oxidative treatment with hydrogen peroxide or potassium permanganate is frequently also carried out subsequently (CARLOS BELLAPART VILA, 1964, ES 297969).
However, all precipitation methods have grave disadvantages. Apart from the relatively high technical costs, the primary disadvantage consists in a material loss of up to 20% of the lactic acid due to the precipitation and crystallization processes (HEDING, L. G., Biotechm. Bioeng. 17, 1975, 1363-1364). Moreover, these methods require large quantities of auxiliary chemicals. Since the lactic acid occurs in most cases as calcium lactate which must first be converted to free acid with sulfuric acid and an equivalent amount of gypsum or calcium sulfate, the costs for disposing of large amounts of calcium sulfate are added to the cost for lime and sulfuric acid.
The grade of lactic acid which can be produced by this "precipitation method" is edible-grade lactic acid and is accordingly only suitable for the foodstuffs industry. However, the pharmaceutical industry requires lactic acid of higher purity. Even higher criteria for purity are required for plastics produced from lactic acid by polymerization. In particular, this requires a total absence of carbohydrates. Therefore, there has been no lack of attempts to find other methods for extracting the lactic acid from the fermentation solution in order to produce higher grades of lactic acid (pharmaceutical-grade lactic acid, plastic-grade lactic acid).
One possibility for producing pharmaceutical-grade lactic acid is steam distillation with superheated steam under vacuum. Lactic acid, as is well-known, has a very low volatility with steam at 100.degree. C. However, the steam volatility can be considerably increased by using superheated steam in a temperature range of 160.degree.-200.degree. C. Based on these results, methods for purification of lactic acid by steam distillation have been worked out, e.g., as described in patents DK 83589 (1957) or CS 97136 (1960). However, this method--by far the oldest--for production of pharmaceutical-grade lactic acid has not been successful in practice since this purification process is much too costly due to the relative "nonvolatility" of the lactic acid.
Liquid-liquid extraction of lactic acid with organic solvents, however, has been more successful. In principle, the procedure in this method consists in that the fermentation solution which has been freed of biological matter is acidified with sulfuric acid, the precipitated calcium sulfate is removed by filtration and, finally, decolorizing is carried out with activated charcoal and salts are removed by ion exchange. The crude lactic acid solution produced in this way is then concentrated under vacuum to a determined concentration and is brought into contact with an organic solvent in a countercurrent extraction column. The lactic acid can then be extracted from the organic phase either by backextraction with water or by distilling off the organic solvent. Further treatment of the pure lactic acid solution with activated charcoal and ion exchangers is often required after the extraction process before it can be concentrated to the conventional commercial concentration of 80%.
A process of this kind is described, e.g., by JENEMANN (1933) in U.S. Pat. No. 1,906,068, where isopropyl ether is used as a solvent.
An extraction process using nitroparaffin as the organic phase is proposed in TINDALL (1940), U.S. Pat. No. 2,223,797.
Also, in more recent times there has been no shortage of attempts to improve the process for obtaining lactic acid by extraction.
For example, DE-OS 3415141 proposes an extraction process in which butanols or pentanols are used as solvents. The characteristic feature in this method consists in that the liquor containing calcium lactate is acidified with sulfuric acid immediately after fermentation and the obtained suspension which contains calcium sulfate and biomass as solids is brought into contact with the solvent directly in a pulsed countercurrent column outfitted with built-in hydrophobic pieces (e.g., made of Teflon). After the extraction of the aqueous suspension by the solvent, which is preferably carried out at a temperature of 70.degree. C., a solids-containing aqueous phase and a solids-free organic phase are removed from the column. The lactic acid dissolved in the organic phase is finally converted completely into the lactic acid ester by distilling the reaction water at 60.degree.-140.degree. C. (possibly under vacuum). This lactic acid ester can be obtained in pure form by vacuum distillation and is a valuable intermediate product. The esters can be split again into lactic acid and alcohol, as is well-known, so that highly pure pharmaceutical-grade lactic acid can be obtained.
A great disadvantage in all extraction methods consists in that most of the organic solvents used for lactic acid have only a very low distribution coefficient so that very large quantities of organic solvents are required.
However, the distribution coefficient for lactic acid can be substantially improved when a mixture of organic solvents with a tertiary amine is used for extraction. A purification process based on this principle is described, for example, in U.S. Pat. No. 4,698,303 (1987) in which a mixture of approximately 60%-75% isobutyl heptyl ketone and 25%-40% Adogen 364 was proven especially effective as an extraction medium. Adogen 364 is the trade name (Sherex Co.) of a mixture of long-chain (C8-C10) tertiary amines.
However, certain difficulties arise in this purification process in recovering the lactic acid from the organic phase, since this can only be carried out by backextraction with a basic solution (preferably ammonium hydroxide). This means that additional purification steps are required after extraction.
Although the lactic acid obtained by liquid-liquid extraction is substantially free of ash, it does contain other impurities stemming from the raw material. Very pure starter mashes as well as additional treatments, e.g., with activated charcoal, oxidizing agents and ion exchangers, are required to obtain pharmaceutical-grade lactic acid by this method (VICKROY, T. B., Lactic Acid in. Comprehensive Biotechnology, Vol. 3, 761-776, Pergamon Press) (PECKHAM, G. T., 1944, Chem. Eng. News, 22, 440-443).
Therefore, the method most frequently applied for the production of pharmacopeia lactic acid is esterification of the lactic acid with low alcohols (usually methanol) and subsequent separation of the esters by fractional distillation. Numerous methods for the purification of lactic acid by esterification are described in the literature. In a part of this process the crude lactic acid which is reduced to a determined concentration, and to which is generally added an acid catalyst, is exposed to the action of alcohol vapors. For the most part, lactic acid is separated from the escaping vapor mixture as esters of the accompanying substances. The surplus alcohol is separated and fed back in a subsequent rectifying column. The methyl lactate can finally be hydrolyzed again with water to form methanol and lactic acid.
The esterifying reaction of concentrated lactic acid with methanol in the presence of an acid catalyst does not present any difficulties with respect to purification. Thus, DE-OS 1912730 describes a process for the production of lactic acid methyl ester in the presence of an acidic ion exchanger, wherein the ester is obtained in a yield of 82 percent by fractional vacuum distillation.
Purification of the esters becomes more difficult when starting from a diluted aqueous lactic acid solution because the ester can be hydrolyzed again very easily in the presence of water. In U.S. Pat No. 2,350,370, although diluted aqueous lactic acid is esterified with an acid catalyst, the distilled ester is saponified again immediately in order to purify the lactic acid.
DE-OS 3214697 proposes a process for continuous purification of lactic acid methyl esters in which the ester, which is produced by esterification of a diluted lactic acid with an acid catalyst, is first concentrated by partial condensation of the gas mixture occurring during esterification and by subsequent vacuum distillation, and the crude lactic acid methyl ester remaining in the sump or bottom of the first separation column which contains only essentially small amounts of lactic acid is guided into a second separation column for complete purification.
Even though the "esterification method" is currently the only usable method for producing pharmacopeia-grade lactic acid, it still has the disadvantage that large amounts of organic solvent must also be used, which poses a considerable safety hazard and risk to the environment.
For this reason there was an intensification of the search for alternative methods not having the disadvantages mentioned above.
Thus, a number of patents are known which propose that the organic acids be separated by electrodialysis. AT 290441, for example, proposes a process for purification of lactic acid in which a lactic acid which is free from "unpleasant odor and taste" is obtained by a combination of electrodialysis and extraction. In this process, a crude lactic acid solution which has been concentrated to approximately 20% is subjected to electrodialysis treatment and the dialyzed solution is then extracted with an organic solvent (isopropyl ether is recommended). The lactic acid is obtained from the organic phase by backextraction with water.
A process for purifying lactic acid is suggested in EP 0393818, in which the lactic acid salt (e.g., NH.sub.4 lactate) contained in the fermentation liquor is first separated by conventional electrodialysis. The lactic acid salt extracted in this way is then directed to a second electrodialyzer which is outfitted with bipolar membranes and in which the lactic acid salt is separated into the free acid and its corresponding base by hydrolysis. Finally, the lactic acid solution is guided through a strongly basic and a strongly acidic ion exchange resin in order to remove any anions and cations which may be present. A lactic acid of high purity is obtained in this way.
However, processes using electrodialysis have the disadvantage that they require large amounts of electrical energy on the one hand, which renders the process very expensive, and additional purification steps, e.g., ion exchange, on the other hand in order to produce highly pure lactic acid.
Another alternative method for extracting and/or purifying carboxylic acids produced by fermentation is the ion exchange method with acidic and/or basic ion exchange resins.
DD-PS 203533 describes an ion exchange process for extracting carboxylic acids and hydroxycarboxylic acids from their solutions containing foreign salts, in which the salts are first transformed into acids via strongly acidic cation exchangers in H.sup.+ form. In so doing, a mixture of carboxylic acids and foreign acids is obtained. A weakly basic anion exchanger which is first present in the form of a base or hydroxyl is loaded with a small concentrated fraction of this mixture, i.e., substantially converted to the form of carboxylic acid. Next, a more highly concentrated fraction of the acid mixture obtained in the decationization step is guided via the acid-charged ion exchanger. The acid to be extracted passes freely through the resin bed and only the stronger foreign anions (chloride is mentioned) bind to the resin by ion exchange.
EP 0135728 describes a process for "isolating enzymatically produced carboxylic acids" in which the solution which is advantageously produced by continuous fermentation runs through a "desorber" filled with a "polymer with tertiary amino groups" which selectively adsorbs carboxylic acids. The liquid exiting from the "desorber" which is extensively free of carboxylic acids is guided back into the reactor again. After exhausting a "desorber", the reaction solution is guided to the next desorber and the carboxylic acid is eluted from the exhausted desorber by means of a polar solvent, e.g., methanol. The separation of the carboxylic acid from the eluate is carried out by known methods, e.g., by distillation in the case of volatile eluting agents.
Other processes for separating lactic acid by ion exchange are suggested in U.S. Pat. No. 3,202,705 and JP 91183487. In these processes, after separation of the precipitated CaSO.sub.4, the fermentation liquor which is acidified with sulfuric acid is first guided through a strongly acidic cation exchanger in H.sup.+ form and then through a basic anion exchanger. In this way, a "colorstable" lactic acid is obtained, at least in the U.S. patent.
The great disadvantage in all methods in which a "genuine" ion exchange takes place is the required cost for regenerating the resins.
Therefore, chromatography processes on basic anion exchangers are described as the most up-to-date alternative method for extracting and purifying organic acids.
For example, EP 0324210 proposes a purification process in which a citric acid produced by fermentation is purified by adsorption at neutral, nonionic, macroreticular, water-insoluble resins or at weakly or strongly basic anion exchangers. Water, a mixture of water and acetone, or a diluted sulfuric acid solution are used as eluent. This method is capable of separating salts and carbohydrates from the citric acid.
The process proposed in EP-OS 0377430 works on precisely the same principle. In this method, basic ion exchangers are also suggested for chromatographic isolation and/or purification of acids. The chief difference compared to EP 0 324 210 consists in that this method can be used to separate and/or purify not only citric acid, but also other inorganic acids (e.g., phosphoric acid) and organic acids (tartaric acid, malic acid or lactic acid).
However, all ion exchange chromatography or IEC methods using basic anion exchangers or nonionic adsorber resins have the great disadvantage that the acids retained on the resin by adsorption show a high degree of "tailing" when eluted with water or with diluted sulfuric acid, which results in an intensive dilution of the eluted acid. Further, it is impossible with this method to separate acids having similar pKs values, e.g., lactic acid and ethanoic acid.